1. Field of the Invention
The present invention relates to an image forming apparatus which forms an image on a recording medium by an ink jet system.
2. Description of the Related Art
In general, in an image forming apparatus of an ink jet system, a plurality of recording heads are arranged at an equal interval, and ink is spouted (eject) to a recording medium, for example, an image forms to a recording sheet by ink spouts. For example, to form a color image, at least four colors of ink are used [black, cyan, magenta, and yellow].
FIG. 14 is a diagram showing a schematic constitution example by four recording heads and a medium conveying mechanism, for forming the color image in a conventional image forming apparatus. FIG. 15 is a diagram showing an image forming timing of image data in each recording head.
In the image forming apparatus, fixed recording heads for discharging four colors of ink are arranged at an equal interval in order of black (K), cyan (C), magenta (M), and yellow (Y) from an upstream side on an upper side of a conveying path of a recording medium in the medium conveying mechanism. That is, each recording head is offset, disposed, and driven/controlled by one data controller. A transmission type inlet TOF sensor (medium sensor) 61 for detecting the recording medium is disposed on the upstream side of the recording head K. A transmission type outlet TOF sensor 62 is similarly disposed on a downstream side of the recording head Y. Front/rear end portions of a recording medium 63 (63a, 63b) conveyed on the conveying path of the medium conveying mechanism are detected by these sensors 61, 62.
In this constitution, when the image is formed on the recording medium 63, the recording medium 63a, for example, recording sheets are taken out of a recording medium cassette (not shown) sheet by sheet, and conveyed to a recording position in a conveying direction X at a constant speed by the medium conveying mechanism. At this time, as described later, each color image data is distributed to each color and input into the respective recording heads K, C, M and Y from a data controller.
Moreover, when a tip of the recording medium 63a is detected by the inlet TOF sensor 61 shown in FIG. 14, different colors of ink are successively spouted to thereby form an image at a predetermined timing by the respective recording heads K, C, M and Y. The recording medium 63a on which the image has been formed is conveyed as such, a rear end of the recording medium is detected by the outlet TOF sensor 62, and thereafter the medium is spouted out of the apparatus, and stored in a containing cassette (not shown) or the like.
The color image data in this constitution is input into the controller shown in FIG. 14 from a host apparatus (not shown) during the image formation.
In the data controller, the color image data is divided into color image data for the respective colors, and transferred to the corresponding recording heads K, C, M and Y respectively. Moreover, the data controller is synchronized with the recording medium 63 conveyed in a Y-direction, the color image data is successively formed into the image on the recording medium 63 by the respective recording heads K, C, M and Y, and the color image data for one page is formed into the image.
The respective recording heads K, C, M and Y are arranged at an interval corresponding to an offset amount L in the Y-direction.
FIG. 15 shows the recording of the color image data transferred to each recording head, and a relation with an image forming time in an image data portion in each recording head.
A time t11 in FIG. 15 is a time obtained by dividing an offset amount L1, between the first recording head K and the last recording head Y in a conveying direction by a movement speed of the recording medium. A time t12 shows a time obtained by dividing an interval L2 between the rear end of the preceding conveyed recording medium and a tip of the following conveyed recording medium by the movement speed of the recording medium. It is assumed that the movement speed of the recording medium is constant. Therefore, lengths of the times t11 and t12 relatively suggest distances.
It is indicated in FIG. 15 that white data corresponding to the interval of the offset amount L shown in FIG. 14 is added to each color image data.
This white data indicates that the recording head does not actually spout any ink.
Moreover, in FIG. 15, color image data 1K, 1C, 1M, 1Y for one page transferred by the data controller, and color image data 2K, 2C, 2M, 2Y for two pages are shown with respect to each of the recording heads K, C, M and Y arranged at the interval of the offset amount L shown in FIG. 14.
When the data controller transfers the color image data for one page to the respective recording heads K, C, M and Y each white data corresponding to the offset amount L in the arrangement interval of the recording heads K, C, M and Y is added to the color image data for one page, and accordingly the color image can be formed on the recording medium 63 without any color shift.
Moreover, after ending the forming of the image by the recording head Y in a most downstream, the color image data of the next page (second page) is transferred to the respective colors of the recording heads K, C, M and Y, and each color image is successively formed with respect to the next conveyed recording medium.
Additionally, each of the above-described recording heads K, C, M and Y comprises one recording head extending along a transverse-width direction X of the recording medium 63.
On the other hand, for example, one recording head K is sometimes constituted by connecting a plurality of head units K1, K2, . . . as shown in FIG. 16. These recording head units K1, K2 are arranged, for example, in two rows (K1 and K2 rows) in such a manner that any gap is not made in the conveying direction Y of the recording medium 63 at an image forming time, and the units are alternately arranged in such a manner that an end portion of a nozzle row 71 of each head unit is slightly superimposed on another end portion.
Even in this recording head, the color image can be similarly formed without any color shift in consideration of an offset amount L3 between the head units (nozzle rows) and an offset amount (distance between the recording mediums) L4 between the recording mediums, and accordingly a color image can be formed without any color shift.
In the above-described conventional image forming apparatus shown in FIGS. 15, 16, after ending the image forming by a detection signal of the outlet TOF sensor 62, that is, the last recording head Y, by the control of the driving of one data controller, the transfer of the color image data including the white data is restarted with respect to all the recording heads K, C, M and Y, and the image is formed from the image data of K.
That is, the color image of each color is formed by the recording heads K, C, M and Y. Therefore, after ending the spout of the ink by the recording head Y of the most downstream, the color image data of the next page is transferred to each recording head to thereby form the image.
Therefore, the number of recording mediums on which the image can be formed once is limited to one sheet.
That is, the medium conveying interval of the recording medium 63 is influenced by the interval between the recording heads in the medium conveying direction, and the interval between the nozzle rows. When the interval between the recording mediums cannot be easily reduced, the number of sheets subjected to the image forming per unit time, that is, throughput does not rise.
In a case where there is an offset amount (distance between the nozzle rows of the recording heads K, Y) L1 of the recording head, a distance (medium conveying interval) L2 between the preceding conveyed recording medium 63 and the following conveyed recording medium 63 has to be set to be larger than the offset amount L1, that is, L2>L1 (t12>t11) as shown. Accordingly, as to a time interval between the preceding conveyed recording medium 63 and the following conveyed recording medium 63, at least a time t11+time for forming the image by the recording head Y is required, even if a time required for forming the image is short.
Moreover, in the recording head in which two rows of head units K1, K2 are arranged alternately forwards/backwards in the conveying direction as shown in FIG. 16, an offset amount (interval between the nozzle rows) L3 between the head unit K1, K2 rows has to be considered in order to reduce the image forming time. When the offset amount L3 is assumed, an interval (medium conveying interval) L4 between the preceding conveyed recording medium 63a and the following conveyed recording medium 63b is set to be larger than the offset amount L3 between the nozzle rows as shown, that is, L4>L3. That is, after ending the spout of the ink by the head unit K2 row, each recording head unit cannot accept the image data of the next page, and the image cannot be formed. Therefore, when there is a relation L4>L3, a first (first page) image is formed on the previous recording medium 63a, and thereafter the second (second page) image data can be formed with respect to the subsequent recording medium 63b. 
Moreover, in general, as to the recording head, even when the head temperature changes by an elapsed driving time, and the recording head is driven with an equal driving voltage, an amount of spouted ink differs, and therefore the density (color depth) of the formed image differs. Therefore, a driving characteristic, that is, driving voltage is adjusted in accordance with the temperature of the recording head. In general, when the temperature of the recording head rises, the ink spout amount increases, and the concentration of the image increases. Conversely, when the temperature drops, the density of the image tends to decrease.
With regard to a timing to adjust the driving voltage, as shown in FIG. 16, in an apparatus which ends the forming of the image on the first (first page) recording medium 63a, and thereafter forms the image on the next second (second page) recording medium 63b, the driving voltage of the recording head is adjusted for each image data.
Moreover, since the image data formed into the image by each recording head differs in a constitution capable of individually driving the recording heads, the density of the image cannot be adjusted while controlling the driving voltage all at once. In an example of FIG. 17, the density can be adjusted after forming a yellow image by the recording head Y disposed on a most downstream side.